Scotland Yard chiefs agreed a secret amnesty for more than a thousand officers and staff who misused corporate credit cards, it can be disclosed.

Senior officers were so overwhelmed by the number of cases they decided the card holders would not be punished.

Instead the staff, the vast majority of whom worked in counter terrorism and other specialist crime inquiries, were given "training and guidance".

More than 300 people were initially referred to anti-corruption detectives when evidence emerged their claims were potentially fraudulent.

Of these cases, 50 were passed to independent investigators. Three officers have since been convicted of criminal offences and two more await trial.

Jenny Jones, of the Metropolitan Police Authority, said training and guidance is "not enough" and some disciplinary action should be taken.

"I find it unacceptable that the police have just let these officers go with guidance. They must have known what they were doing was wrong."

Members of the MPA, the force watchdog, have been told privately how an initial sift of accounts exposed the scale of the problem.

Internal auditors decided 1,183 people who used a card, but did not break the law, should be put to one side and labelled "category B".

These cases included those who purchased personal items and later repaid the money or bought equipment that should have been purchased by other means.

An internal MPA document stated: "It was agreed between all interested parties that due to the volume of files involved, those officers that are deemed to have category B files would receive no formal discipline sanction for their card use, but would receive 'training and guidance' with regards their use."

Meanwhile, claims found to be "potentially unacceptable", including cases of suspected fraud, were labelled "category A" and passed to anti-corruption detectives.

Inquiries into abuse of the credit card system by officers are expected to continue until next March, two and a half years after they first began.

A total of 3,533 officers and staff were issued with the cards and at one point £3.7 million of public money was unaccounted for.

The majority of this money has been paid back, but legal action is expected against two officers who owe £82,000 and £1,100.

The Met did not renew its American Express contract and has introduced a new Barclaycard system with tighter controls.

Ms Jones added: "Personally I feel that guidance is not enough, some sort of disciplinary action ought to be taken, even if it just a note on their record.

"Having police who do not obey the rules is damaging for public confidence in the Met because you ask: 'What other rules do they break?'."

An MPA spokesman said: "The MPA internal audit initial review of potential cases of business card misuse identified 1,183 officers who had made purchases outside the policy for use of the cards.

"These fell into two categories: claims outside of the business card policy i.e. an officer should have used an alternative procurement process to purchase particular items of equipment or services for operational use.

"And, claims outside of the business card policy that contained items of personal expenditure but were repaid by the officer within acceptable parameters.

"These purchases were contrary to the rules and while the officers were not subject to formal discipline sanctions they received training and guidance.

"The Met contract with Amex has since expired. A new contract has been introduced with robust rules and accompanied by extensive staff training to reduce misuse."

An IPCC spokeswoman said: "The IPCC agreed with the Met that any cases where there were possible misconduct or criminal offences committed would be referred to the IPCC, all other cases would be dealt with locally by the Met.

"To date 50 such cases have been referred to the IPCC. Of those, three officers have been convicted, two are awaiting trial, 14 have been given written warnings, one has received words of advice, one has received a formal reprimand and two are awaiting misconduct hearings."